Universal joints are used to transmit rotational motion between angularly misaligned shafts. A common universal joint used in these circumstances is the Cardan joint. There are, however, many applications where a Cardan joint is not well suited, because of a requirement of very wide angle operation, a requirement for constant velocity operation, or both. One common solution to this is to use a double Cardan universal joint, so phased and aligned as to provide constant velocity operation over a wider angle.
One configuration that yields constant velocity operation in angularly displaced shafts is when the center yoke divides the angle between the driving and driven shafts, and the axes of rotation of the driving and driven shafts are maintained in a coplanar configuration. See, for example, "On the Necessary and Sufficient Conditions for Homokinetic Transmission in Chains of Cardan Joints", by D. A. Johnson and P. Y. Willems, ASME Journal of Mechanical Design, June 1993, Vol. 115, pp. 255-261, which is herein incorporated by reference. It is this configuration which most of the prior art utilizes.
In some configurations in the prior art, extensions of the end yokes mesh in the center of the double Cardan joint to provide constant velocity alignment. This meshing is accomplished in a variety of ways. One approach uses a ball on the end of one end yoke extension and a cylindrical socket on the end of the other end yoke extension. This particular approach is confined to relatively small angles, has lubrication and wear problems with the ball and cylinder, and does not produce precise alignment over the full range of use. In another approach disclosed in U.S. Pat. No. 4,352,276 (Smith), the extensions of the end yokes are equipped with meshing annular gear teeth. This provides precise alignment, but again the angle of deflection is limited and the teeth sustain considerable wear. Additionally, the arrangement suffers from instability at greater angles of deflection when the teeth become somewhat worn. In still another invention employing this concept, and disclosed in U.S. Pat. No. 4,257,243 (Herchenbach), there is an intermediate movable piece with which the end shaft extensions mesh in a variety of configurations. This approach claims to be effective at up to 90 degrees of total deflection and produce precise alignment, but at the cost of considerable sliding surface with the attendant wear and frictional losses. Collectively, this group is characterized by either a restricted angle of operation, considerable sliding surfaces, or both.
Another group of inventions uses external guides to give double Cardan joints a wide angle, constant velocity capability. U.S. Pat. No. 1,389,970 (Noel) purports to provide a wide angle of operation, but is restricted to a single plane. U.S. Pat. No. 4,236,420 (Geisthoff) attempts to overcome this by adding another Cardan universal joint and a spring mechanism to prevent direct alignment from causing catastrophic failure if movement occurs in the wrong direction. This approach requires considerable space and hence is proposed primarily for connections between tractor and implement, where such space is available. This approach does not provide the constant velocity capability required in such applications as front end drive vehicles.
Wholly new types of universal joints have been proposed; and some, such as the tripot universal joint, have won widespread acceptance in the automobile industry. They have, however, suffered from restricted angles of operation, sudden failure at high stress and deflection, and failure to produce true constant velocity operation. The strength, reliability, durability, efficiency, wide angle capability, and the potential for true constant velocity operation continues to make the double Cardan joint a desirable alternative.
There remains a need in the art for a way to guide double universals over wide angles without being constrained to a single plane for true constant velocity capability, for reduced frictional losses, and for reduced wear.